This invention relates to a method of using a nitrogen oxide trap comprising platinum, barium, and incorporating cesium, zinc and/or a combination of cesium and zinc to absorb nitrogen oxides during lean-burn operation, release the nitrogen oxides when the oxygen concentration in the exhaust gas is lowered and provide a NOx trap with significantly increased sulfur poisoning resistance.
Catalysts are employed in the exhaust systems of automotive vehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides (NOx) produced during engine operation into more desirable gases. When the engine is operated in a stoichiometric or slightly rich air/fuel ratio, i.e., between about 14.7 and 14.4, catalysts containing precious metals, such as palladium and rhodium, are able to efficiently convert all three gases simultaneously. Hence, such catalysts are often referred to as xe2x80x9cthree-wayxe2x80x9d catalysts.
It is desirable, however, to operate gasoline engines under xe2x80x9clean-burnxe2x80x9d conditions where the A/F ratio is greater than 14.7, generally between 19 and 30,to realize a benefit in fuel economy. Such three-way catalysts are able to convert carbon monoxide and hydrocarbons but are not efficient in the reduction of NOx during lean-burn (excess oxygen) operation. Efforts have been made in developing lean-burn catalysts in recent years. One deficiency of some of the conventional lean-burn catalysts is that they are based on zeolite materials which are less than durable at the elevated temperatures necessary for their efficient catalytic operation in the exhaust gas system. Lean-burn catalysts act to reduce NOx through the use of hydrogen, hydrocarbons and carbon monoxide over a catalyst.
Recent efforts to solve the problem of NOx in lean-burn systems have focused on lean-NOx traps, i.e., materials which are able to absorb nitrogen oxides during lean-burn operation and are able to release them when an oxygen concentration in the exhaust gas is lowered. Hence, these traps are used with engine systems which operate primarily in a lean air/fuel ratio, but then when it is desired to purge the traps of NOx, the exhaust entering the trap is made richer, particularly rich of stoichiometric conditions. Typical catalyst materials used in conventional traps are alkaline earth metals like barium combined with a precious metal catalyst like platinum. European Patent Application No. 0613714 A2, published Sep. 7, 1994, discloses that platinum or palladium in various combinations with at least two ingredient materials of the alkali metals, alkaline earth metals, transition metals, or rare-earth metals are capable of storing or absorbing nitrogen oxides under exhaust conditions of excess oxygen and desorbing the NOx during stoichiometric or fuel-rich atmospheres. When the NOx is purged, it is expected that the NOx is reduced over the precious metal to nitrogen and oxygen containing species such as carbon dioxide and water.
The widely held mechanism for this absorption phenomena is that during the lean-burn operation the platinum first oxidizes NO to NO2 and the NO2 subsequently forms a nitrate complex with the other material, e.g., the barium. In the regeneration mode as during a stoichiometric or rich environment, the nitrate is thermodynamically unstable, and the stored NOx is released. NOx then catalytically reacts over the platinum with reducing species in the exhaust gas like HC, CO and hydrogen to form N2 and CO2 and H2O. According to one strategy for using lean-NOx traps, a hybrid-mode engine strategy is used to cycle the air/fuel ratio between extended periods of lean operations where the traps sorb NOx emissions, alternated with brief, fuel-rich intervals to desorb the adsorbed NOx and regenerate the lean-NOx trap. U.S. Pat. No. 5,473,887 discloses such operation of an exhaust purification device, the teachings of which are hereby expressly incorporated by reference herein.
The alkali metal and alkaline earth metals which are typically utilized for NOx sorption have, however, the serious drawback that they are readily poisoned by sulfur in the exhaust gas. Most fuels for automotive vehicles contain sulfur and when burnt, the sulfur is converted to sulfur compounds like SO2. Over time, the sulfur compounds react with these alkali metal or alkaline earth trap materials forming sulfates which will not revert back to the sorption material. The available sites for NOx sorption and desorption are reduced and NOx conversion efficiencies are reduced. As a result, the typical NOx trap material which uses precious metal and an alkaline earth, such as barium, is strongly deactivated by sulfur in the fuel.
According to the present invention, we have now found that the incorporation of cesium and zinc into the platinum/barium oxide/alumina trap provides significantly increased sulfur poisoning resistance.
This invention is directed to a method for treating exhaust gas emissions which comprises locating a NOx trap material in an exhaust gas passage of an internal combustion engine, exposing the trap material to engine exhaust gases having a lean of stoichiometric air/fuel ratio whereby the trap material absorbs nitrogen oxides from the exhaust gas; monitoring the amount of absorbed nitrogen oxides in the trap; and purging the absorbed nitrogen oxides by subjecting the nitrogen oxide trap to engine exhaust gases whose air/fuel ratio is stoichiometric or rich of stoichiometry. The NOx trap material comprises: (a) porous support material comprising mostly xcex3-alumina; and (b) metals consisting essentially of 1-5 wt % of (i) a sulfur-suppressing metal such as cesium, zinc or a combination of cesium and zinc, (ii) a precious metal such as platinum, and (iii) a NOx sorption metal selected from the alkali metals (Group 1A), alkaline earth metals (Group IIA), and light rare earth metals such as cerium and praseodymium, the amount of the metals being individually based on the weight of the support material. The trap may include other precious metals like rhodium or palladium. Preferably, the support material is xcex3-alumina, and platinum, barium oxide and the sulfur-suppressing metal are loaded on the support sequentially with the sulfur-suppressing metal being loaded after the platinum and barium.
Advantageously, we have found that the incorporation of cesium and/or zinc into the NOx trap provides significantly increased sulfur poisoning resistance. It is believed that the interaction between platinum and these metal oxides suppresses the oxidation of sulfur dioxide over the precious metal, promoting the desorption of sulfur containing compounds, i.e., sulfites and sulfates, and resulting in improved NOx trap sulfur tolerance. This allows for improved traps as compared to the prior art conventional traps.